texts: Stellar nurseries in the "Shedding New Light on the Whirlpool Galaxy" video

texts:
Captions in the Hubble Space Telescope YouTube channel video Shedding New Light on the Whirlpool Galaxy say:

(visible) Along the dark dust lanes, bright pink nebulae are the birthplace of new stars.
(X-ray, lower energy) Before leaving these stellar nurseries, the most massive stars die in supernova explosions, which heat the surrounding gas to X-ray temperatures.

I'm not very familliar with star formation processes.
These might be considered as separate questions, but I've put them here together because they all refer to the same two sentences in this video, and it's likely that those familliar with nurseries, or wanting to know more about them, will have something to say or learn about all three.

Do stars tend to leave "these stellar nurseries" after a while, and it's only the short lifetime of the most massive stars that keeps them from leaving before going supernova?
What impact does the heating of surrounding gas to X-ray temperatures have on the formation and evolution of other stars within a given "nursery"?
Are there really such things as spatially well defined "nurseries"? Are they distinct and roughly countable, or is this a subjective popular term? Can one point to something and say "this little blob here at x, y, z in the spiral arm of the Whirlpool Galaxy, it's a classic nursery"?


texts: If a black hole does not emit light, how can one take a picture of the black hole itself?

texts:
There's some discussion that the image composed by the Event Horizons Telescope is really just an accretion disk. To "take an image" of something, you need the light reflecting off of the surface of the object within frame. The "light" in the image is really just color-mapped gases within the accretion disc. Since black holes do not allow light to escape, is this actually an image of a black hole?

The analogy I've heard is that it would be like taking a picture of an object in a completely dark room, showing someone the photograph taken, and saying, "Hey look at this picture I took of this object." The other person could ask, "Where is the object in this entirely black photo?" to which the other person might respond, "It's somewhere in the picture."

Was a picture of the object really taken if you can't even see it? If you photograph the sky, have you "captured an image of air molecules?"


texts:
A couple of years ago I started playing a game called Kerbal Space Program (KSP) and my understanding about how orbits actually work increased dramatically. Because of KSP I was looking at a picture of a galaxy with its spiral arms and something struck me as very odd. The stars closer in should orbit the galaxy center faster than the stars farther out and that would smear out these lanes of stars until they were no longer recognizable. Something weird is going on here!

So why do galaxies have arms? What's bunching these stars together like that?

I noticed that the center of galaxies are all smeared out and bulbous like I think that the rest should look like. So what if all that mass rotating around a common center is warping space around it, like the water draining down your sink, and this warping is what catching matter/stars into the lanes.


texts: Is the moon's revolution speed increasing or decreasing

texts: I've come across multiple answers to this question. So, the moon is moving away from the Earth a couple of centimeters per year and the Earth's rotation is slowing down. Here it says the moon's revolution is speeding up, another place it says it is slowing down. So, due to tidal effects, is the moon's revolution slowing down or speeding up?


texts: What if you 'placed' the Chicxulub asteroid onto the Earth?

texts:
We all know what sort of devastating impact an asteroid could have if it collided with the Earth.
But I'm curious to know what the effects would be if you removed the speed element from it, and were somehow able to casually 'place' a giant asteroid onto our planet?
I think I read somewhere that the Chicxulub asteroid was most likely around 9 miles long, and that the end of it would have extended beyond our atmosphere if it hit the Earth lengthways.
But would this have any effect if it was travelling at, say, 2mph? There would be nothing burning in the atmosphere or shockwaves etc.
Would it have any affect at all apart from squashing whatever was underneath?
If the answer is no... how about placing a moon-sized object onto the Earth?


texts:
Suppose the Sun rises and due to refraction the Sun's disc gets deformed. Let the center of the sun be $O$ and $OL$, $OR$, $OT$, $OB$ be the distances from the center of the disc to the edges in the direction $L$eft, $R$ight, $T$op and $B$ottom ($L$, $R$, $T$, $B$ are the farthest points from the center of the disc to their direction due to the deformation of the solar disk). How to find which of the 4 distances is bigger and which is smaller?
Thanks in advance for your answers


texts:
I'm working on a star map program and I'm wondering what are the most common map projections used to print star maps...? I want to map of the entire visible sky from a certain point on the surface of Earth. I have all the visible stars and their Altitude-Azimuth, but I need to map this semi-sphere onto a circle in 2D.

I'd like to get something like this:

I've been playing around with equidistant and gnomonic (-1 RE) projections but they feel too distorted. What are the most common projections used for 2D, circular, entire-visible-sky star maps?


texts:
So it seems the overall thought is that the universe is infinite. I don't get how.

Most depictions I read mention something to the effect of "at 1^-20 seconds the universe was the size of _". (Essentially mentioning a finite size at a very small length of time). So it came from a singularity, and had a finite size at some point. No matter how fast inflation went, any multiple of that finite size is NOT infinity though. If it was finite once, it is still finite.

If it's infinite then it was always infinite which means it was infinite density, infinitely large, etc. That's too crazy to swallow.

Isn't it more likely that the universe is really really big but not infinite? And we are at the center of the observable universe (to us), however we are off center in some grander universe that's maybe 1 trillion light years across? And nothing is past that because space and time are the same thing. Therefore if there is no light/matter/events, there is no time and hence no space.


texts: Can an Earth-like planet survive if our Sun went Supernova?

texts:
First, I would like to point out, yes, our sun does not have enough mass to be a candidate for Supernova, this is a scenario were our sun is though, and after the various life cycle stages of the sun and whether or not life managed to survive or not on the planet, The Sun goes of as a Supernova, it is 1 AU from an Earth-like planet, or equivalent to 1 AU from a massive star, would the planet still be standing or would it be obliterated from the face of the Galaxy?

Here are some of the links that I have been looking through but could not find an answer for:

https://www.physicsforums.com/threads/effective-destructive-range-of-supernovae.312925/
https://earthsky.org/astronomy-essentials/supernove-distance
https://www.popularmechanics.com/space/deep-space/a26483/supernovas-deadly-twice-as-far-away/


texts: How can I collimate a dobsonian telescope with a laser collimator?

texts:
I've had a dobsonian telescope for a while now, but I've never been able to collimate  it. I thought I had it collimated once, but when I looked through it the star didn't look right, it looked kind of like a round disc with a hole in the middle. I know the basic idea behind doing it with a laser collimator, you need to get the laser to line up with the center of the primary mirror as well as line up in the center of the target on the laser itself. However, I always have a lot of difficult accomplishing this.

For starters, I always have a lot of trouble getting the secondary mirror lined up. I'll get really close, but once I get close I have a lot of trouble doing the fine tuning to get the laser in the middle. I'll get it in, but when I got to tighten the other screws it will no longer be lined up. I'll try to account for the tightening as well, but it's always a guessing game for me when it comes to knowing which screws to loosen and tighten so that the laser winds up where I want.

I don't have that many problems with the primary mirror, and can normally get that lined up fairly easily. However, usually after lining up the primary mirror the secondary mirror no longer seemed to be lined up and I need to do the secondary mirror all over again. I'm assuming this is to be expected, but considering the difficulty I have lining up the secondary mirror it's very frustrating.

I've tried looking around at resources online, but none of them go into enough detail into aligning the secondary mirror. They just say "adjust the screws until it's lined up", which doesn't solve my problem, since I have trouble figuring out exactly which screws to adjust and when. I'm also confused as to what I should see through my telescope when it is properly aligned. Most of the sites just use a diagram, but I'm never really sure what to make of the diagram. The one time I did line it up it looked like a round disc with a hole in the middle, which I didn't think looked right.

So to summarize, the main problems I have are:

I have trouble fine tuning the secondary mirror, since I have trouble accounting for the movement of the mirror when tightening the two screws I didn't loosen
I'm not sure what a star should look like through my telescope when it is properly collimated

Can anyone offer some detailed instructions, and possibly a video, of how to collimate a dobsonian telescope with a laser collimator?


texts: So I'm a beginner at astronomy and I was trying to look at the moon but while trying to aim at it I saw a ring-like object in the sky. Does anyone know what this would be? all I could see was a ring with nothing in the middle of it. Was this a special type of star?


texts: Would all neutron stars inevitably collapse into black holes due to quantum tunneling?

texts: The materials inside a neutron star are densely packed (extremely close to each other) and as time goes by say about an eon later, most of the neutrons would eventually tunnel into each other and voila! the neutron star becomes black hole? sound suspicious to me but can it happen?


texts: What is the temperature of the solar atmosphere (the corona) and how is it measured?

texts: The temperature of the Sun's atmosphere, also referred to as the solar corona, is known to be hot even hotter than the Sun's surface. What is its temperature and how is it measured? How does it compare to other stars and how does it vary across the Sun?


texts: We know that last year, space X rocket took orbital reflector with it into the space. But what might be the reason behind launching this weird object into the space? Does it do any work there?


texts: Kepler - Creating the Ellipse (Astronomia Nova)

texts:
In Chapter 58 in Astronomia Nova, Kepler made his final step to finally create the ellipse (for the orbit of planets)
http://science.larouchepac.com/kepler/newastronomy/part4/58/index.html

In the given source there is written:
"Dissatisfied with so large an error, Kepler writes that he rejected his hypothesis of chapter 56, which was supported by the distances of chapters 51 and 53 as well as the physical arguments of chapter 57"

For his hypothesis of chapter 56 he made use of the distances of chapters 51 and 53. And now, he rejects this hypothesis. Does that mean the distances he calculated were wrong? Or did he just use these (correct) datas for a wrong suggestion for orbits?

Here you can see the determination of the distances:
http://science.larouchepac.com/kepler/newastronomy/part4/51/index.html


texts: Red Moon a Characteristic of all Total Lunar Eclipses?

texts: I understand that the redness of the moon during the total lunar eclipse last night/this morning (April 14-15 2014) is a reflection of the red light caused in earth's atmosphere due to Rayleigh scattering (basically, a reflection of all the sunsets and sunrises happening around the earth).  Because of this, it seems to me that the moon will appear red during every total lunar eclipse that happens.  Is this the case, or are there some lunar eclipses where the moon does not turn red?  If it is the case that all total lunar eclipses produce a red moon, why was the media so hyped up about last night's eclipse producing a red moon?  Was it especially red?


texts: How powerful a telescope would allow me to view the astronauts aboard ISS do a space-walk?

texts:
This arose from a comment posted against a question elsewhere on the stackexchange

How powerful a telescope/binoculars would allow me to view the astronauts aboard the ISS do a space-walk? Aperture? Magnification?


texts: How long does it take Dawn to orbit Ceres?

texts:
I haven't been able to find the orbit time of Dawn around Ceres.
Do you know how long it takes?


texts: How can lunar pits be distinguished from craters?

texts:
ESA's news En route to exploring lunar caves features the following paragraph:

The Moon's surface is covered by millions of craters, but it also hosts hundreds of very steep-walled holes known as pits. Like doorways to the underworld, photos of some pits clearly show a cavern beneath the Moon's surface, suggesting that they are 'skylights' into extensive lava tubes [...]

I am now wondering how those pits have been detected. Reading  the Is seeing the apollo moon Landers via earth telescope that hard? it looks like that Earth-based telescopes could be ruled out, and observing Lunar pits most probably requires a lunar orbiter, correct? Is there a list of lunar pits (including their locations) available somewhere? Does it require exact laser or radar topographic measurements of the surface of the Moon, or is it possible by optical observations alone? Could radar-astronomy be used to find lunar pits?


texts: How much of the Sun's disk must be covered for a visible shadow to be cast?

texts:
At first it seems like the answer is obvious, $100$% or more, that's what a total solar eclipse is after all. But after seeing satellite images of the June 21st 2020 annular eclipse, the shadow cast by the Moon on Earth was indistinguishable from the one cast during a total eclipse despite the Sun not being completely covered. Only difference is that there was no umbra, but that doesn't matter.

But then again, during an annular eclipse the Moon is covering about $99$% of the Sun's disk, and that's a lot. Surely if less of the Sun's disk was covered then there would be no shadow visible. But then I learned Phobos, which covers ≈$25$% of the Sun's disk as seen from Mars' equator, casts a very clear shadow despite covering only a quarter of the Sun's disk.

  What exactly is the cutoff point for a visible shadow to be cast then? Venus only covers $0.108$% of the Sun's disk during a transit, and thus no visible shadow is cast on Earth. If Venus was larger, say the size of Jupiter, it would cover $14.52$% of the Sun's disk. Would that be enough for a visible shadow to be cast on Earth's surface? Granted, it wouldn't be an umbral shadow, but that's irrelevant.


texts: How can I see a star behind a mountain range? Do I move apart from it and up?

texts:
Could some explain what I need to do to see a star that, from my point of view, is behind a mountain:

Obviously, if it's one mountain, and not a very high one, I can just climb it to see what's behind. But what if it's a mountain range or a very high mountain?

Intuitively, I know I can move away from it to change the angle, and see the star. But what's the math behind it? And how does going up (or down, maybe?) help?


texts: How can light reach us from 14 billion light years away?

texts:
One thing that I can't quite wrap my head around is how light is traveling to Earth from 14 billion light years away, aka the beginning of the universe.

The way I see it, the universe itself was very small 14 billion years ago and has been steadily accelerating in size since.  So the photons that left a star that existed 14 billion years ago couldn't travel for 14 billion years because the universe itself was probably only a few million light years total in diameter.  The universe is not expanding faster than the speed of light, so that photon should have hit the end of the universe before now.

What piece of this puzzle am I missing?


texts: Why aren't all planets in the same plane?

texts:
Obviously all planets are not in same perfect plane. Because if at all they were in the exact same plane, it would mean that Mercury transits and Venus transits would not be that rare.

Any explanation would be much appreciated.


texts: Measuring star distance by parallax using a small telescope

texts: I was wondering if it's possible to measure star distance using parallax at home with a small telescope (Something like a 3-inch reflector)? Or is this not really practical?


texts: Why is 3GHz the best frequency at which to measure the sun's microwave output?

texts:
I have read that 3GHz is the best frequency at which to measure the sun's microwave output, but that measurements are made at 2.8GHz for historical reasons, however I can find no explanation of why 3GHz should be the best frequency.
Could anyone tell me please?


texts: What must/do astronomers reveal beyond their academic papers?

texts:
Academic papers often make claims about observational data and results from computations. The papers only refer to names of data sources (such as an observatory) and of mathematical methods without any specifics, other than some of it illustrated in a few graphs and tables. References lead almost only to other papers of similar format.

In order to replicate (or even peer review) a paper, obviously one must have access to specifics such as the actual calculations, the raw data, the software, the blue prints of the measurement equipment and logs of how it has been operated.

What data do peer reviewers and replicating scientists (and the public) have access to beyond a paper making some claims?

Two examples: The results of the infamous cold fusion press conference of 1990 could never be satisfactorly replicated. But were the attempts based only on the paper they then published, or did they publish on the side also a wealth of data about their procedures? And the more recent "neutrinos travel faster than light" media debacle was, as far as I know, all about the details of equipment, was all relevant information about it made public (for the science community)?


texts: Is it likely that intergalactic stars would still retain their planets?

texts:
Given that we've discovered hundreds of intergalactic stars (IGs), and that most stars have planets, what do we know about the likelihood of IGs retaining their planets after being ejected from their galaxy?

Related to that, what are the chances of rogue planets being intergalactic as well? If there's a planet floating out in intergalactic space, is it more likely to be orbiting a star, or on it's own?


texts: How does hawking radiation work without virtual particles?

texts: How does hawking radiation work without the existence for virtual particles? I read that certain quantum fields are distorted when a black hole forms on top of them. But what fields specifically? If hawking radiation is thermal radiation then it should be the photon field that is distorted, right?


texts: how to measure temperature of the distant star

texts: As part of small project i want to measure temperature of the distant star on basis of optical observations. so by what exact method i can do it? which instruments will be necessary? what is the precision of the experiment?


texts: Could spy satellites use laser guide stars (for adaptive optics)?

texts: Are sodium lasers useful for Earth observing space telescopes/spy satellites?


texts: How is Comet SWAN C/2020 F8 doing? Will it become easily visible by eye?

texts:
The New York Times article How to See Comet SWAN in Night Skies says:

  Astronomers have their fingers crossed that the comet will keep brightening in the coming weeks as it heads north, passing 52 million miles from Earth on May 12 at its closest approach to our planet, and then rounding the sun on May 27.

and links to the comet's twitter account which is tweeting non-specific updates every few hours.

On May 12, 2020 https://minorplanetcenter.net/mpec/K20/K20G94.html puts the comet at

Date    TT    R. A. (2000) Decl.     Delta      r    Elong.  Phase   m1    m2
2020 05 12    01 17 42.4 +14 52 01   0.5572  0.5849    28.5   124.4   4.4

about R.A 1h 17m and Dec +14° 52' and an estimated magnitude of about +4.4 (if I'm understanding this correctly) and the JPL Horizons database puts it at

 Date__(UT)__HR:MN, Date_________JDUT, , , R.A._(ICRF), DEC_(ICRF),  Azi_(a-app), Elev_(a-app),   T-mag, N-mag,             delta,     deldot,     S-O-T,/r,     S-T-O,

2020-May-12 00:00, 2458981.500000000,*,m,    19.54137,   15.02829,   105.324535,    61.907026,      9.,  n.a.,  8.3196461568E+07, -3.4857438,   28.4079,/L,  124.6636,

But I'd like to ask if Comet SWAN C/2020 F8 is likely to become easily visible by eye at any point in time during this pass near the Sun.


texts: How far into intergalactic space would you have to go to see the Milky Way?

texts: If in the distant future a ship were somehow able to move beyond the galaxy and into intergalactic space, how far would it have to go before we could look out a window and see the Milky Way in its entirety, rather than just edge-on as we see it from Earth?


texts: Astrophysics of Supernovae - Energy over Distance?

texts:
I need this for worldbuilding, but it is a physics question. I want to be able calculate the energy over distance from a hypernova of a variable star like Eta Carinae. Understanding what energy levels would be dangerous to a planet, and dangerous on what level, would also be necessary.

Scientists have numbers saying that a normal supernova would damage Earth from within 50 to 100 light years. I'm more interested in hypernovae, because Eta Carinae is 7500 light years from Earth, and some scientists are saying it could strip our ozone layer literally any minute, day, year, or century now. I have to wonder: if it might be that bad here, how bad is it 5000 light years or 1000 light years away?https://en.wikipedia.org/wiki/Eta_Carinae#Possible_effects_on_Earth

I know we have models to predict these, but after repeated attempts at searching I cannot find them, and maybe I just don't know the correct terms to search. I like this answer to a similar question: https://worldbuilding.stackexchange.com/a/19002 but it appears to be about planets orbiting the dying star having a normal supernova.

I'd like to be able to take the characteristics of the star and create a distance spectrum that goes from planetary evaporation to an aurora light show for a wide range of planets: rocky, Neptunian, or Jovian . Being able to estimate that of any star would be a bonus, but I mostly just want to be able to do massive variable stars.


texts: Impact on the rest of the solar system if Mercury or Venus were disassembled?

texts:
I've watched a few theoretical videos about Dyson spheres where they mention that the most practical way to do that would be to disassemble a planet - preferably one already close to the sun - Mercury or Venus.
Question: Would the solar system become completely unstable if one (or both) of those planets got disassembled? As a follow up question: what if one or more of the gas giants somehow were taken out of the solar system?


texts: We know that with the passage of time, hydrogen inside (near or at the core) a star is converted to helium and other heavy elements via fusion. We can then deduce that the old sun contains heavier elements in comparison to the new one. Does that mean that the mass of the old sun is more than the new sun?


texts: What's the safe distance from a hypernova?

texts:
Hypernovae are even rarer than supernovae, occuring in stars at more than 30 solar masses, destroying the star that goes hypernova. This post states a hypernova releases several million times more light than all of the Milky Way’s stars put together.
A safe distance from a supernova from Earth is considered 50-100 light years. How far would a hypernova have to be from Earth to not cause significant damage to the Ozone layer, and how far to not destroy all life on Earth?


texts: The Sun formed 4.5 billion years ago, in a molecular cloud. I assume that there were other stars in the vicinity (as is common in molecular clouds). Which stars are they? Where are they now? Are they similar to the Sun?


texts:
The introduction to Testing the rotating lighthouse model with the double pulsar system PSR J0737-3039A/B (open access) says:

The double pulsar system PSR J0737-3039A/B was discovered by Burgay et al. (2003) and Lyne et al. (2004). This system consists of a 22-ms pulsar (hereafter A) and a 2.8-s pulsar (hereafter B) with an orbital period of 2.4 h. This discovery has provided a laboratory for the study of relativistic gravity and gravitational radiation (Kramer & Wex 2009). The system has several strange features that challenge the current understanding of pulsars and provide an uncommon opportunity to improve pulsar theories. One of the most interesting properties is the observed modulation of each pulsar's signal by the energy flux from the other, as evidenced by each pulsar's modulation period being approximately equal to the other pulsar's pulse period (McLaughlin et al. 2004a,b). Freire et al. (2009b) proposed a technique for analysing the arrival times of the pulsars’ pulses and their mutual modulations which could yield the sense of rotation of each pulsar with respect to its orbital motion, among other quantities.** In this paper, a complementary technique is presented with the same objective, but using measured periods rather than arrival times. The principal benefit of our approach is that it is simpler and more intuitive. If the validity of either of these techniques is confirmed, not only will new insights be gained but also the correctness of the lighthouse model will be empirically assessed beyond dispute.

I think there is a lot going on here, more than I can take in all at once. So I'd like to simply ask:
Question: What exactly is "the rotating lighthouse model" in the context of a double pulsar? Modulation is mentioned in this and the other papers, and an interaction with each others' magnetospheres is suggested to explain it, but is that part of "the rotating lighthouse model"?


texts: Do the E and B modes of the CMB polarization have anything to do with electric and magnetic fields?

texts:
As explained in this article, as the CMB is created from a vaiable (matter?) density $\kappa$ its gradient $\nabla \kappa = \vec{u}$ can be separated into a gradient and curl part, called the $E$ and $B$ modes

$$
\nabla^2 \kappa^E = \nabla \cdot \vec{u}
$$

and

$$
\nabla^2 \kappa^B = \nabla \times \vec{u}
$$

Thinking about electromagnetism, these definitions look slightly (but not exactly) familiar, if the vector $\vec{u}$ would take the role of the electromagnetic potential $\vec{A}$.

Is the similarity to the terminology of electromagnetism just a coincidence, or do these $E$ and $B$ modes have indeed something to do with electric and magnetic fields ?


texts:
There are millions of galaxies out there in the entire universe. When a telescope is set to a direction to to identify the stellar objects, it can capture all lights from all the objects. How do we identity that the particular light is emitted from one star/galaxy and not the other one?

Even if we precisely point to one star, there are chances that other luminous objects interfere with the light ray.


texts: Create Position-Velocity diagram from a velocity field

texts: Can someone explain to me how I can generate a Position-Velocity (PV) diagram from a galaxy's radial velocity field? I understand that usually PV diagrams are generated across the major axis of the galaxy, but I am not quite sure why they appear as a distribution. For example:


texts: I have some questions about star formation

texts:
Q2 What does the phrase 'containing several distinct knots of collopsing material' mean?

Q3 In this context, does the word 'such clouds' mean protostar? (I'm not an English-native speaker.)

Q4 In the context, does the word 'a spinning disc' mean an Accretion disk ?

Q1 And the last... Could you explain 'spiral density wave' to me if you can?


texts: How much star can star fusion fuse if star fusion can fuse hydrogen?

texts: During stellar nucleosynthesis a star will convert a portion of its hydrogen atoms into helium.  Is there a constant of how many tons of hydrogen is converted based on the mass of the star?  Is it based on a percentage of the star's mass? Or can it vary? A Nasa blurb stated that 600 million tons of hydrogen per second are consumed by our own sun. Is there a mathematical formula that can tell you how much any star would consume and convert to helium?


texts: Are a black hole's jets caused by the black hole's magnetic field?

texts:
Do the jets sometimes emitted by a black hole result from its magnetic field?  It would seem Jupiter's magnetic field tends to be concentrated at the poles, and I would imagine this might extend to black holes as well.  I could see the concentrated fields near the poles accelerating particles either into or away from the black hole at high speeds.  I have included an image of Jupiter's magnetic field below:

http://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter#Role_of_Io

It also appears that a black hole's magnetic field is indeed very strong:
http://www.iflscience.com/space/magnetic-fields-can-be-strong-black-holes-gravity


texts: Simple rule to find some of the zodiac constellations after sunset?

texts:
How do I find some of the constellations of the zodiac without remembering their locations on the sky?

I think it should be quite simple if you are located in the northern hemisphere and you know
1) which astrological sign it is right now (this month)
2) where the sun set
3) the number of hours left until sunrise
4) the order of the constellations in the circle of the zodiac: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces.

Straight forward logic says that it is enough to make calculations in order to understand where they should be, but I can't find any such rule on the internet. Maybe it is not so simple. Have you seen such a rule?


texts: How to estimate the inclination of a galaxy between edge on and face on?

texts: I have the images of a galaxy and the field of view and I would like to determinte the inclination of a galaxy between edge-on and face-on. Would that information be enough to do the calculation? Or what other details would be needed?


texts: What is the highest granularity focal-plane array on a dish radio telescope? Or is this the ONLY ONE?

texts:
There is a short Wikipedia article Focal Plane Arrays that enumerates some projects, but my question is more along the lines of what is (at least) nearly complete or in "first light" phase, even if not commissioned yet.

I'd like to differentiate between focal plane arrays (an array of multiple feeds and amplifiers) used in single dish telescopes, and those integrated into multiple dish arrays, because I'm particularly curious about single dishes being used for spatial information, or even true imaging. Ideally, the answer will give some information for each case.

In the case of single dish instruments, are the elements - roughly speaking - used as pixels? Despite the longer wavelength, it's still optics and it is a telescope. If there are N individual, uncoupled feeds, does one build up an image roughly N times faster? Is the relative phase between the feeds ever used (for single dish instruments)?

For those like me who aren't already familiar with focal plane arrays, here is a random picture from one of the links I found in a quick internet search. It's from the Parkes 21cm Multibeam Receiver, has (had?) 13 receivers and sat at the focus of the 64m dish. The photo is dated 1997 - I have a hunch there's been some development in this technique in the intervening 20 years.

Is this actually the only one?

Edit: The Parkes array is still in use as shown below:

Above: Superposition of the half-power beam widths of the 13-element array of the Parkes 21-cm Mutlibeam Receiver, as used in a study of a Fast Radio Bursters.

It's likely the image is a screen shot from from The host galaxy of a fast radio burst Nature volume 530, pages 453–456 (25 February 2016), Keane et al. I can't find my archived copy now, but instead see Phys.org's New fast radio burst discovery finds 'missing matter' in the universe

where the caption there reads:

  This image shows the field of view of the Parkes radio telescope on the left. On the right are successive zoom-ins in on the area where the signal came from (cyan circular region). The image at the bottom right shows the Subaru image of the FRB galaxy, with the superimposed elliptical regions showing the location of the fading 6-day afterglow seen with ATCA. Image Credit: D. Kaplan (UWM), E. F. Keane (SKAO).


texts: Do any particles in AGN jets escape the galaxy?

texts:
I have read, at http://www.thestargarden.co.uk/Black-holes.html for example, that whole stars can be ejected from certain galaxies. "These are thought to have been part of a binary star system that broke apart as it approached a supermassive black hole. As one star was captured, and the other was pushed away at a velocity exceeding the escape velocity of the Galaxy."
My question is about the jets that can form at the poles of the rotating supermassive black hole: "At the rotation axis of the supermassive black hole, matter from the accretion disc can be pushed away at the speed of light, creating jets that can extend for thousands of light-years."
At thousands of light-years away, is this matter still gravitationally bound to the galaxy?  (If not, maybe some particles can go around in a big loop that takes a while.)


texts:
Procrastinating, I clicked on a suggested video (How Large Can a Telescope Be?) and after about 01:20 I saw a clip of a telescope shown below.

I was confused as I saw the reflections in the mirror until I decided that there are actually two small objects in front of the primary mirror, one far from the primary where you'd expect to see a secondary, which I've labeled "A", and one very close to the primary, which I have labeled "B".

Both are supported by four vanes, and there does not seem to be a hole in the primary.

I've labeled the reflected image of the back side of B as B' (B prime), and I've indicated the reflections of an "A vane" and a "B vane" with arrows.

But "A" looks very small compared to what I'd expect, only perhaps a few percent of the primary's diameter, and it looks like the telescope is not in a conventional dome, but perhaps has only a sliding or otherwise openable roof (currently off screen) to protect it when not in use.

Question: How does this telescope work, and what is it for? Is it for imaging, or performing some other function?

https://www.youtube.com/watch?v=3wOFAkggSiU


texts: Can mean motion resonances make orbits more unstable rather than more stable?

texts:
This paper on closely packed orbit simulation seems to suggest that having the planets in first and second order resonances decreases the time till orbits cross each other. I'd got the impression that resonances were greater in stability the lower order they were, but if I'm reading this right, it seems like they get the most time in their simulation if they space each adjacent pair of planets at a certain multiple of their mutual hill spheres without regard to resonant ratios.

So if we had a system of planets in say 2:3 resonances through each pair, that would actually be more unstable (less time till orbit crossing) than if the planets were non-resonantly spaced? That seems backwards to how I understood this, though they do concentrate on the overlap of resonances, so perhaps its different for a whole chain of bodies rather than two. Then again, real systems like Trappist-1 exist which seem to be in long chains of near resonances or at least groups, so I'm not sure I understand this.


texts: Why do we need to add a second to 30 June and risk upsetting Internet?

texts: We linked a physical constant (the time of one second) to a physical non constant (the rotation of the earth) so there will have to be adjustments - or not? Is it important that the UTC in one line on earth is the same as the astronomical time? As the earth slows down, it means that this line slowly changes position. When the line travels 1 hour away we can bring it back by an hour change if necessary. If currently we add 1 second say every 3 years, it would take thousands of years to change by 1 hour. The last time a second was added, in 2012, problems were caused in some systems. Why do we need to do it?


texts: Likelihood of a stable system with a dwarf planet's orbit inside that of a gas giant

texts:
I keep thinking of various planetary system configurations, and would like to know:

What are the fundamental references based on Monte Carlo long-term simulation of planetary systems' evolution that put testable restrictions on the distribution of stable configurations around Sol-like stars (books, papers, software packages)?
How rare is the configuration wherein a dwarf planet (Ceres, I'm looking at you) is located in the inner system?

The main question is the first one, in the vein of "teach a man to fish...".

Related Q&A's here that do not answer my question:

 Is there a upper limit to the number of planets orbiting a star?
 Can you explain the pattern of Hill sphere sizes of the objects of the Solar system?
Could a habitable satellite of a gas giant have a stable subsatellite?


texts: Vectorial construction of tidal forces - or why is it centripetal at low tide

texts:
I am seeking to understand why the tidal force is pushing towards the center of the earth (centripetal), at a point that is making with the center of the earth a 90º angle to the moon-earth axis.

This is very well seen in the common picture (from the Wikipedia "Tidal force" article):

I remember a vectorial construction of that from the French edition of Scientific American ("Pour la Science"), in their August 2001 issue. But I lost it, and it is now behind a pay wall:

https://www.pourlascience.fr/sd/geosciences/les-caprices-des-marees-4441.php

I used to have the physics/mathematical background to understand that, and perhaps I could dig it out of the decade-old sediments of my memory. The vectorial construction looked interesting to me as slightly easier to explain to people around than the differential equations.


texts: Is there any evidence that the Milky Way could have been a quasar in it's early history? Is it thought that most galaxies come from quasars?


texts: How to detect emission lines in optical spectra?

texts:
Is there any handy module to detect emission lines in a spectrum like one we get from the Sloan Digital Sky Survey (SDSS)?

You can see there are many emission lines like Ha,OI in the spectrum below. In fact the spectrum is a two column data set, wavelength and flux. It is a combination of lots of scatter points. The typical interval between two points nearyby is 1.5 angstroms. I need to find all the Gaussian bumps and their line center.

So,first I need to find the the bumps and then I need to fit them to get their centers.


texts:
I plan to buy a 12" or 14" dobson, the choice depending on my final budget and on the total weight of the instrument. I have several options for the primary mirror. I tried to find a formula that would give the acceptable range for mirror thickness depending on the diameter, maybe also on the material (BK7, pyrex, ....). Is there such a formula, or a back-of-the-enveloppe calculation, which could help me? If not, what do people go with usually (for these diameters, I mean)?

Thank you for your help.


texts: Relationship between Periapsis and apoapsis longitude

texts:
What is the relationship between Periapsis and Apoapsis longitude ? Are they 180° apart ? (ie) Periapsis + 180° = Apoapsis ?

Related question: if apoapsis is not known, what orbital elements are required to compute it ?


texts: How can the IAU 2000A vs IAU 2000B nutation comparison be reproduced?

texts:
I am attempting to plot how the IAU 2000A nutation model degrades as its terms are omitted. As a spot-check, I decided to compare it to IAU 2000B, which includes only the 77 most important lunisolar terms. It is widely claimed that IAU 2000B only sacrifices 1 mas of accuracy. For example, in this PDF presentation from SOFA:

  The IAU 2000B nutation series is almost as accurate (1 mas) as the full
  IAU 2000A series

http://www.iausofa.org/publications/aas04.pdf

And in this published paper:

  IAU 2000B is more than an order of magnitude smaller than IAU 2000A but achieves 1 mas accuracy throughout 1995-2050.

https://www.aanda.org/articles/aa/full/2008/04/aa8811-07/aa8811-07.right.html

But I have not yet succeeded in comparing them in such a way as to produce a difference that small. When I compare the angles they return each day over the two decades 2000–2020, I see a difference in the first angle — delta psi — of >2 mas. Using the USNO NOVAS implementation of the two series, because it was easier to install (pip install novas) than pysofa, I get:

from math import tau
from novas.compat.nutation import iau2000a, iau2000b
T0 = 2451545.0  # Year 2000.0

dpsi_differences = []
deps_differences = []

for day in range(0, 366 * 20):  # Years 2000.0 through ~2020
    dpsi_a, deps_a = iau2000a(T0, day)
    dpsi_b, deps_b = iau2000b(T0, day)
    dpsi_differences.append(abs(dpsi_a - dpsi_b))
    deps_differences.append(abs(deps_a - deps_b))

def report_difference(name, differences):
    radians = max(differences)
    days = differences.index(radians)
    degrees = radians / tau * 360.0
    arcminutes = degrees * 60.0
    arcseconds = arcminutes * 60.0
    mas = arcseconds * 1000.0
    print('Maximum difference for {}: {:.4f} mas at T0 + {} days'
          .format(name, mas, days))

report_difference('delta psi', dpsi_differences)
report_difference('delta epsilon', deps_differences)

Result:

Maximum difference for delta psi: 2.1867 mas at T0 + 1396 days
Maximum difference for delta epsilon: 0.8631 mas at T0 + 7017 days

Am I misinterpreting the output of the NOVAS routines? Or, alternately, am I misunderstanding the meaning of the two angles? I understand the angles as being a pair of rotations which each, in the worse case — that of a point on the great circle of the rotation — will move a coordinate through the same angle as the rotation itself. So I understand a 2.1867 mas difference in Δpsi, for example, as changing sky coordinates by a maximum of that same 2.1867 mas when the nutation matrix is used to translate coordinates into or out of the equinox-of-date.

My next step would be trying to get the sofa library installed locally and then running a similar routine against it in case the NOVAS implementation is simply broken, but before trying to install a library by hand, I wanted to double check in case my understanding of the angles was itself faulty.

Thanks for any snags that can be identified in my reasoning!


texts: When will the next 'Super Blue Blood Moon' occur?

texts: I missed the sight yesterday (1 feb 2018) because I live right in the region where the sight was not visible. So when will the next Super Blue Blood Moon occur?


texts: What is the naming convention for temporary moons?

texts:
Is there a naming convention for temporary moons? Or do they follow the naming conventions of other small bodies?
In the case of 2006 RH$_{120}$, 2015 HP$_{116}$, 2020 CD$_3$ and 2020 SO, the first number is the year. I assume the following letter+number combination somehow indicates the time of year of discovery and/or the number of similar bodies discovered prior?
How were YX205B9, WT1190F, S509356 and XC83E0D named?


texts:
Some planets, such as Mars, have 2 or more moons. Giants, like Jupiter of Saturn have a lot of moons!

How likely the orbit of one crosses another one's orbit? Is it possible for two moons to collide? If yes, what would exactly happen?


texts: Does the size of the atom limit the focal length of telescopes?

texts:
Does the size of the atom place a theoretical limit on a telescope's focal length (and thus, resolution)?

It seems to me that that the allowed margin of error for parabolic mirrors drops with an increasing focal length. It's already like 25 nm for advanced Earth-based telescopes - which is just 50-250 atoms.

If we try building mirrors with an ever increasing focal length (e. g. as a part of a huge space-based interferometer), will we eventually reach a point where the precision required will be less than the size of the atom and thus no longer achievable? If yes, how far away are we from this limit now?

If we had unlimited resources, would it be possible to construct a space-based telescope for imagining extrasolar planets in great detail, or would this be not even theoretically possible due to the fixed atom size?


texts: Apparent Magnitude / Apparent Brightness relationship help

texts:
I'm a high-school student that is currently working on a project that describes, using mathematical notation and explanation, how scientists can find the values of each of the properties of stars, such as Luminosity, Temperature and Mass. Currently, I'm struggling with finding the equation for the apparent brightness and apparent magnitude of a star. I have stumbled across something called the "Pogson relation", but need help it a couple of things.
I found the description of how to use it in another post:

The easiest way to determine the magnitude of a given star is probably to use the Pogson relation. The idea is to determine the magnitude of a star knowing the magnitude of a reference star; it is thus quite easy, using a well-known reference as Vega or Sirius.
The Pogson relation is given by:
$$m_1-m_2=-2.5\ log\ \left({\frac{E_1}{E_2}}\right)$$
where $m_1$ and $m_2$ are the magnitudes of star 1 and star 2 (your reference star), and $E_1$ and $E_2$ brightness

Firstly, is there any version of this equation which doesn't have a comparison between two stars, but rather a direct relation so that given $M_{\text{Apparent}}$, I can find $E$ of the star?
Secondly, if not, then what I had trouble with was the 'well-known reference' part of the explanation. It said that I use a value for $E_2$ that is known. However, after researching for the value of the apparent brightness, or its other name flux, for both Sirius and Vega, I have come up empty-handed. Is there a place that I can find this? I've looked at Wikipedia, even consulted the Gaia database, but in the field, there was nothing, just the apparent magnitude.
Lastly, is there any way to directly measure the apparent magnitude of a star with a scientific instrument, like how a CCD camera is used to measure the apparent brightness?
Thank you.


texts: Could Venus be a source of Earth's apparent overabundance of water?

texts:
I've watched documentaries about the solar system, where it is suggested that Venus once had oceans of liquid water similar to those that cover most of Earth today.  Venus is now in a period of runaway greenhouse effect and has lost its oceans.

I've always wondered whether the apparent overabundance of water on Earth could be partially explained by our planet sweeping up the water, pushed into Earth's path by the solar wind, after it had been liberated from Venus.  I know one theory on where our water came from is a bombardment of comets - but I've always wondered whether some could also have arrived after Venus heated up.

How stupid is this as an idea?

edits:-

10/12/2014 - article from the bbc - Rosetta results: Comets 'did not bring water to Earth'

20/09/2017 - article from www.spacetelescope.org - Hubble discovers a unique type of object in the Solar System which includes an interesting note - "[2] Current research indicates that water came to Earth not via comets, as long thought, but via icy asteroids."


texts: Are there currently any planned or proposed missions to measure the gravitational fields of other planets in our solar system?

texts:
I just recently found out about the GRACE Nasa mission which basically mapped out the gravitational anomalies of earth as well as serving extensively in the study of earths geology and the movement of its distribution of water. While I continued reading about it I came across GRAIL which did a similar job but over our moon.

Link here to learn about GRACE: GRACE

Here for GRAIL: GRAIL

What i'm now wondering is are there space agencies or organizations considering missions to any of the other planets in our solar system and attempting to measure their gravitational anomalies in a similar manner hopefully giving vital information on their inner structure or formation.


texts: If we know that the reason a star burns without oxygen and fuel is that it converts hydrogen into helium and we have these two elements on earth then why can’t we make a star on our planet


texts: Why are three year old eclipse glasses not recommended?

texts:
NASA's eclipse safety page says that eclipse glasses should:

  Not be used if they are older than three years, or have scratched or wrinkled lenses

Why shouldn't three year old glasses be used? Is it because the coating somehow degrades with time? Were standards changed three years ago such that the older glasses can no longer be recommended? Is there just a stronger concern for scratches or damage over the years? Will eclipse glasses (or other solar filters) purchased today be unsafe three years from now?


texts: Could the Sun and our Solar System one day leave our galaxy?

texts: I have now learned that the Sun is not fixed in place, so I asked myself this question: It is possible, that the Sun and our Solar System will one day leave our galaxy? If yes, how, and if no, why not?


texts: Why are tides biggest during the equinoxes?

texts:
I've read that during the equinoxes, tides are at their maximum. Shouldn't this only apply to latitudes near the equator? I mean, when the solar declination is near to 0º and we have a full or new moon, the combined effect of the Sun's and Moon's gravity should create the highest tides, but only near the equator (see in wikipedia the article Tides, which says that equinoctial tides are maximum)

Is this true for other latitudes?


texts: Is there an upper limit on the mass of black hole mergers we can detect?

texts:
From the LIGO website, black hole mergers have been observed between black holes with a mass up to roughly 50 $M_\odot$.

Are there no black holes with a mass above 100 $M_\odot$ or is this an observational bias? Why haven't we observed any mergers between black holes with a mass in the 100 - 1000 $M_\odot$ range?


texts: Could diatom blooms affect albedo of an exoplanet?

texts: And perhaps display a seasonal variation if the exoplanet has an axial tilt, providing proof of an Earth-like world supporting at least oceans with microbial life?


texts:
I'm wondering why there is a shadow under the Yutu 2 rover, of the Chang'e 4 mission, since it is on the far/dark side of the moon.

The Chang'e 4 lander, as seen by the Yutu 2 rover. (Image credit: CNSA)

See the Far Side of the Moon in This Amazing Panorama from China's Chang'e 4 Lander


texts: V838 Monocerotis "light-echo" images morphed into nice video, but why so few original images?

texts:
This NYTimes video shows what I think is a series of real Hubble images which have been "morphed" artistically to appear to be continuously expanding. (also see The V838 Monocerotis Star Still Has Astronomers’ Heads Exploding) If I understand correctly it is the light-echo, not actual material that is doing the expanding at this rate. They also give four discrete images available in 2006 (shown below) and you can find a link to a video there which also seems to be made of just four images.

This is (very roughly speaking) tomography of the cloud - the parabolic "shell of reflection" expands and each image highlights a view of the material in that shell. That seems to be what the diagram illustrates.

my question is: why weren't there more images taken of this - technically speaking - awesome event, enough to make an animation without the artistic "morphing"?

note: I'm not asking "why do you think there weren't...". I'm looking for the actual reason. Are these "found images" - nobody thought of doing it and later just pulled these from archives, or where these thought to be sufficient? Are there more images available?

Here are some links to video I have found

YouTube:
"Hubble: Timelapse of V838 Monocerotis (2002-2006)"

https://www.youtube.com/watch?v=U1fvMSs9cps

...which can also be seen at spacetelescope.org

http://www.spacetelescope.org/videos/heic0617a/

... and this Wikipedia article links to this video

https://upload.wikimedia.org/wikipedia/commons/1/1b/Evolution_of_the_light_echo_around_V838_Monocerotis_%28Heic0617a%29.ogv

and here are some "stills"

Images from NYTimes


texts:
We need to figure if liquid water is present throughout a Mars year, and how long the planet has been at its current distance from Sun. That will give us the length of time that liquid water has been present.

We can then compare this with Earth's data to figure out how long life would have had to evolve and what state it may have reached. In particular would there have been enough time for humanoids to evolve?

Would the different composition of the atmosphere and surface gravity produce life that looks different to that on Earth today?


texts: How much does the sky change in a few thousand years?

texts:
The "fixed stars" are not actually fixed, the earth's tilt changes over time etc., but all that happens slowly on human timescales.
Imagine a Babylonian astronomer (or astrologist?) teleported to the present, e.g. to present-day Iraq. Would she be able to tell that the sky is different? If so, what about a Maya  astronomer? In other words, how long does it take until the changes in the sky are perceptible to the human eye?


texts:
Is the albedo of any of the Trappist-1 planets know? Is it high enough so if you are on the surface of one of the planets, you can see the others?

I looked up the distances from the star and size of the planets and it seems a few of them will appear larger than the Moon in the night sky, unless the albedo is really low that is.


texts: Portion of universe visible if gathering image from inflationary epoch

texts:
Apologies in advance for my over-exposure to pop science.  I want to propose a telescope powerful enough to collect imagery from 10^-32 seconds after the “Big Bang”, or as soon as the end of the inflationary epoch.  With light coming from the universe when roughly 10 ly in diameter, along with the expansion to our current size, I would only see the tiniest fraction of light from so long ago with a telescope looking at that far of a distance.

Assuming my telescope has an identical view field as the Hubble Space Telescope, what fraction of the total volume of the universe at end of the inflationary epoch would I actually be viewing?


texts: Why do M dwarfs emit more X-ray radiation than larger, more luminous stars?

texts: I have read that "The relative amount of harmful radiation (to life as we know it) that stars emit can be 80 to 500 times more intense for M dwarfs relative to our Sun". Source This seems counterintuitive to me since the HRD depicts M dwarfs to be less luminous. How are these types of stars able to produce such copious amounts of radiation? What are the physical mechanisms behind this? I cannot find an answer to this online or in papers. The information I find simply restates it but does not explain it. I could use some insight as this seems counterintuitive.


texts:
Using the Python package Skyfield I thought that I could get the RA expressed in degrees simply by doing ra.degrees, but I get the following message

this angle is usually expressed in hours, not degrees; if you want to use degrees anyway, then please use the attribute _degrees

I tried a few things, but cannot find a solution. Could anyone help me?


texts: Why do tides reach a peak two days after the solar eclipse?

texts:
On June 21st we, in India, experienced a total solar eclipse. Based on my basic understanding of why tides occur, the tidal forces by moon and sun add up and the tides will be maximum on the day of eclipse.
But to my surprise, tides kept on rising for next two days and reached a maximum on June 23rd.

It doesn't make much sense to me. Can anyone please clarify.


texts: Ignore pulsars and the like, just a neutron star in empty space – does it emit light and neutrinos?


texts: We know that the Galaxies are moving very fast, not only the Galaxies other known and unknown objects in the universe, are moving. But when it was started where is the starting point, and how it move with so much speed. Is there any theory on that  ? Thanks in advance.


texts: How are Y-dwarf spectral subtypes defined?

texts:
It is often stated that the spectral subtype indicates the relative position between spectral types, e.g. G5 being halfway from the start of spectral type G to the beginning of spectral type F.

Given that we haven't observed a spectral type cooler than Y, how can subtypes Y0, Y1, Y2 be defined?


texts: Are radio telescopes finding radio-dark Nebulas

texts: Are there "dark nebula" spots in the sky for radio telescopes?  By that I mean, are there portions in the sky where these telescopes do not receive any radio waves?   If so, do they match the visual dark nebula spots.


texts: Rule of Exception in All Nearest Objects

texts:
I once heard of a strange phenomenon, but now cannot find anything about it on the Internet.  I might be looking for the wrong keywords or misremember some relevant aspect.  So maybe someone here can help me out and point me in the right direction.

The described phenomenon goes like this, as I remember it:

During the course of the scientific research, people found out more and more about the objects surrounding us.  Moon, Sun, planets, other stars, other galaxies, etc.  But each time (more or less) a little later (after a new discovery was getting an established fact) it showed that the closest object of this kind to Earth was rather an exception among its peers.  For instance, the closest other galaxy is blue-shifted.  Or that the closest Pulsar is somehow an exception among its peers.

It might be that the rule was not about the closest but about the first found object of a kind.

I think I remember Prof Harald Lesch (a German physicist lecturing on TV in a popular format called "Alpha Centauri") mentioning this peculiar phenomenon and fleshing it out with some examples, but now I'm not able to find the clip.

I understand the point about closing this question, but maybe leave it open for some weeks (or months), just in case somebody drops by who has something to contribute.  After a grace period the non-reaction can be counted as a vote for "isn't common knowledge".  Without such a grace period closing the question can only be based on "I don't know such a thing" (which should not be enough).

The concrete questions (because that was asked for, so it seemed to be unclear to some people) are:

Is such a phenomenon known?
Is there a source describing it?

Nice to have:  Which of the Alpha Centauri lectures was probably the one introducing me to the topic?

Is there a name to it?
What are examples of such objects?


texts: Sedna, VP113 and the likelihood of the PX/Tyche/Thelistos hypotheses

texts:
Crosslisted question from https://physics.stackexchange.com/questions/131876/sedna-vp113-and-the-likelihood-of-the-px-tyche-thelistos-hypotheses

The recent discoveries in exoplanetary science (specially those findings of planets orbiting far far away from its parent star/stars) raise questions about how much we know about the (true, AIU definition) number of planets in our solar system.

Question: does the discovery of VP113 and Sedna increase the odds of some kind of planetary object in the edge of the known solar system (Oort's cloud)? How could we find it if it is very cold and dark? I mean, how many Sedna or VP113 objects we would need in order to guess its position and location?

Reading about this topic, I don't know how to quantify the odds of finding

PX(Planet X): An Earth sized body perturbing the orbits of Sedna, VP113 and other similar objects.
Thelistos: A super-Earth (about 5-10 Earth masses times a O(1) factor)
Tyche: (Now disfavoured by WISE data, I think it can not be excluded if it is extremely cold and "dark") Gas giant (Neptune-like, Saturn-like, etc)
Nemesis: A "dark star"/brown dwarf object.


texts: Which Barlow lens should I choose, 2x or 3x?

texts:
I recently bought a 6-inch reflector, 750 mm focal length and I want to get a Barlow lens to get better observations. I have 2 eyepieces, one 9mm and one 25mm.

My question is:

  Should I go for a 2x Barlow with the 9 mm eyepiece (167x magnification) or 3x with the 9 mm (250x magnification)?

I know that a 6 inch can handle about 300x. Is the 3x Barlow too much or is it suitable for what I currently have?


texts: If months are based on the moon, then why are the months longer in the Gregorian calendar than lunation?

texts:
What I have always thought / known is that:
Days are based on the period of rotation of the earth.
Months are based on the moon.
Year is the time taken by the earth to make one revolution around the sun.
But if we take a look at the lunation it takes on average 29 days, 12 hours, 44 minutes, and 28 seconds. Why is it that the months then in the Gregorian calendar have 28-29 / 30 / 31 days?


texts: Would dark energy save the earth for a while as the sun heats up?

texts:
Would the expansion of space (e.g. dark energy) move the earth further away from the sun over time, possibly saving us for perhaps a few billion years as the sun heats up?

Scientists now tend to think that the sun is heating up very gradually (e.g. over billions of years):
http://tvblogs.nationalgeographic.com/2010/07/21/hotter-and-hotter/

Dark energy is the unexplained accelerating expansion of the universe, as described here:
http://en.wikipedia.org/wiki/Dark_energy

Put another way, my question is, is dark energy working at such a rate that it would move earth (and other planets) further from the sun and prolong life on earth as the sun heats up?


texts: How is the polar angle of a pulsar beam determined?

texts:
I just read the question "What is a typical polar angle of a pulsar beam?" and I was surprised that it had an answer. I thought there was little real information besides the timing of the pulses, and it seems like any given timing could be consistent with a beam with any cone angle.
As I think about it, if you know the angular width of the beam as it leaves the pulsar, then you could infer something from the relative durations of the pulse and the time between pulses. Is that it? Or is it something else?
Note: I misunderstood the first question to be asking about $\alpha$ in the notation of the accepted answer, but they were really asking about $\rho$, the angular radius of the "lighthouse beam". It is easier for me to understand how that could be estimated.


texts: Has the computed change of the axial tilt ever been measured?

texts:
Earth, the 3rd planet from the Sun, sometimes faces earthquakes and seaquakes that are so strong that they reportedly change the axial tilt and/or the rotation velocity. These changes are merely calculated, but have there also been some measurements/experiments that confirmed those calculations? If so, how did they measure it and due to which earth-/seaquake as the cause for the reported changes?
Since Jovian moon Io faces many ioquakes and eruptions too, I'd also like to know whether that happens on Io too and whether it was confirmed by some probe.


texts: Why does the outer layer of the sun have differential rotation?

texts: I don't feel that way myself but some might find it really strange and wierd that it has differential rotation and feel that it's a contradiction. They may say "How is it possible?" I am asking on behalf of others who might have that question what assumptions they are making and how to break them.


texts: How does the Eddington limit make the gravitational field of stars asymmetric?

texts:
This answer to Is the gravitational field of the sun uniform? says:

Very large stars are asymmetric due to being close to the Eddington limit. The Sun is too small of a star to have the kind of gravitational anomalies that very large stars and rocky planets exhibit.

which is really interesting but leaves me unsatisfied.
Question: How exactly does the Eddington limit make the gravitational field of stars asymmetric?

What does Eddington Limit (Eddington Luminosity) Show?

From wikipedia:

Eddington luminosity
Humphreys–Davidson limit


texts: How to determine arrival time delay given dispersion measure?

texts:
In an online textbook, the following formula is given to calculate the arrival time delay between two frequencies, $v_1, v_2$ in a dispersed radio pulse:
$$t_1 - t_2 = 4.15 \cdot DM [(v_1/\text{GHz})^{-2} - (v_2/\text{GHz})^{-2}]$$
Where does the $4.15$ come from, and what exactly is “GHz”? Do I divide by 1 GHz or?


texts: At what cosmological redshift $z$, does the recession speed equal the speed of light? How is it calculated?

texts:
At what cosmological redshift $z$, does the recession speed equal the speed of light?

What equations are used to calculate this number (since at large redshifts, $z=v/c$ won't apply)?

[The interested reader may refer to answers on Physics SE.]


texts: Is there a paper on galaxy mergers in clusters of galaxies?

texts:
I'm looking for something to cite about galaxy mergers in clusters of galaxies.

I need numbers on the frequency of mergers in the outer parts of galaxy clusters vs. the cluster center. In the center number densities are higher but so is the velocity dispersion.

Are mergers of galaxies more likely to occur in the outskirts or the central region of galaxy cluster?


texts: Would a tablespoon of a neutron star remain intact?

texts: I've heard people say that a tablespoon of neutron star would weigh over a billion tons. If we ever could take a tablespoon of one would it still remain intact with the same density?


texts: Why are asteroids with zero orbital inclination rare?

texts:
This is a plot of orbital inclination ($i_p$) vs. semi-major axis ($a_p$) of 96944 asteroids in the Main Belt, done by Piotr Deuar.

Some structure can be seen in this diagram; clumps are asteroidal families and vertical empty spaces are Kirkwood gaps due to mean-motion resonances with the planets (mosly Jupiter).

Recently I've noticed that there are fewer points towards the ecliptic in the first $\small{\frac 1 2}$ degree of orbital inclination. Is this due to some selection bias? or Is it because of some kind of resonance that eliminates asteroids with orbital inclinations too close to zero?


texts: Are there any alternative explanations for the gravitational force we attribute to dark matter?

texts:
It always seemed counter-intuitive to me that we observed an measured a gravitational force and since the universe has not enough mass to account for it, the conclusion was to say there's a different kind of matter wich doesn't interact with ordinary matter nor can be seen o measured.

What if the force was applied by other adjacent or overlapsed universes or by something completely different? Are there any other theories propossed? or would we just call whatever we find out to be the source of this force "Dark matter" even if it´s nothing alike matter?


texts: Astrochemistry - what does freeze-out mean?

texts: I was recently watching a video on the physics and chemistry in planetary accretion disks, and the speaker at a certain point mentioned 'freeze-out and UV dissociation'. Is freeze-out a purely chemical term, or is it specific for this process? Is it very important in planetary formation?


texts: How to calculate the time and distance of closest approach of a star?

texts:
How should I approach this problem?

A star has a motion on the sky of 0.78 arcsec/yr in declination and 1.33 arcsec/yr in right ascension.
Calculate the velocity of the star in the plane of the sky in km/sec.
The star also has a radial component of its velocity, coming towards earth
at 4.3 km/s. What is the time and distance of closest approach?

I know how to do 1. but I am not sure how to approach 2.


texts: What's the actual speed of electromagnetic radiation in space?

texts:
The speed of EM radiation is very slightly less than $c$, because space is not quite a vacuum.  Say EM travels at $(1-\varepsilon)c$.

For example, this results in a slight delay between receiving a gravitational wave and detecting an associated EM emission from the body that caused it.  For an object about 130 million light years away (GW170817) I've seen this delay quoted as everything from $1.74$ seconds (so $\varepsilon\approx 4\times 10^{-16}$) to 27 minutes (so $\varepsilon\approx 4\times 10^{-13}$).  I'm sure these figures depend on all sorts of interesting astronomy to do with black hole mergers and gamma ray bursts, but the actual value of $\varepsilon$ doesn't need to know any of this.

So I'm asking here because anyone working on LIGO must surely know our best estimate for the value of $\varepsilon$.


texts: What are the analysis steps in taking raw data from Kepler to a planetary system determination

texts:
I wish to get a concise list of the analysis steps required to take raw light data from a Kepler data set of a star through the steps needed to get to an analytical determination of the existence of a planetary system (i.e. find a planet candidate in a light curve).  I would also like any software analysis tools that would be suggested for each data analysis step.  A good reference specifically outlining these steps would also be helpful.

I would first like to replicate the complete set of steps needed to corroborate the existence of a well accepted planetary system. After that I would feel confident that I could move on to analysis of some Kepler KOI data sets that NASA has published or even some data from a non-KOI entity.  I am an amateur and I realize that astronomical knowledge needs to be applied at each data step and that this might not be an automated process but the steps of this analysis would open up new areas of study for me.

I have tried to assemble the basic data analysis tools from various astronomical python sites and other light transit tool sites and get a little familiar with them but I am not clear as to the actual steps as to how these programs feed to each other.  I have looked at the NASA Kepler pipeline to see the steps but it is not clear to me what specific software they apply. I have read several papers confirming exoplanetary discoveries but from them I can't get a complete picture of the data analysis steps and software employed.

It is disappointing that the actual code listings are not presented in the papers or in a separate reference doc.  In the past (50 years ago) , it was required to publish code listings in reports and references to the code listings in any papers.  But, this discussion is probably another subject.


texts: What caused these strange craters on Hyperion?

texts: I was looking at pictures of the moons of Saturn and noticed that the craters on Hyperion have a strange shape, somewhat resembling sinkholes. They look a lot deeper than the impact craters on other moons (including ours). Is it an optical illusion (perhaps caused by the presence of dark material at the bottom of the craters?) that somehow makes them look deeper? If not, what process caused these holes to appear?


texts: Since when are we able to discover interstellar objects similar to 1I/ʻOumuamua?

texts:
Since when are our observation capabilities good enough to discover objects similar to the first discovered interstellar object 1I/ʻOumuamua (October 2017) with high probability?

Could we make rough estimate how frequent are passes of similar objects through the inner Solar System?

What could be the probability that during this century we will be able to gain more observational knowledge about interstellar objects?


texts: Why we can approximate the mass of spiral galaxy like this?

texts:
This is a question from Galaxies in the Universe: An Introduction(2nd edition),Linda S.Sparke&John S. Gallagher III. As shown in the picture
My question is why we can neglect the bulge of spiral galaxy and approximate the mass as $\mathcal{M} \propto V_{\max }^{2} h_{R}$
The equation 3.20 mentioned in this question is:$\frac{V^{2}(r)}{r}=-F_{\mathrm{r}}(r)=\frac{G \mathcal{M}(<r)}{r^{2}}$


texts: What is the etymology of the astronomical term metal

texts: It's well known that Astronomers use the term "metal" to describe any element heavier than Helium but could anyone tell me why? What is the history of this particularly strange bit of language?


texts: In the first episode of Star Trek: Discovery, we learn that Captain Georgiou has an old-fashioned telescope in her ready room. Michael Burnham inherits it a bit later in the season. I wonder if anyone can identify exactly which telescope it is. What's the model and manufacturer? It seems to be an equatorial-mounted refractor with a finder scope.


texts: How can Bond Albedo be greater than Geometric Albedo?

texts:
Have gathered from papers like this one (*) that "Albedo is differentiated into two general types: (1) normal, reflective or geometric albedo, and (2) bond albedo". The document explained that Geometric Albedo usually is bigger than the Bond Albedo, which often just reflects two-thirds of the reflective light of Geometric Albedo.
Wikipedia points out in this article about Bond Albedo; that Bond Albedo sometimes  can be greater than Geometric Albedo, which apparently is the case with Venus and Mars.
Question is; How Is this possible? What is behind this phenomenon? I think Wikipedia is trying to explain it, but I have difficulty grasping it. Can it be explained in an easier way? Examples with pictures would be ideal.
(*) "Background Reading to Albedo Investigation" (Google this document title)


texts: Are the viewable nebulae only in our galaxy?

texts:
Where are all the nebulae located?

Yes, a simple question that should be obvious but apparently is not to me.

To be more explicit, how far are all these objects? Are they mostly in our galaxy?

I'm not suggesting that the Milky Way galaxy is the only galaxy that has nebulae, but that the ones we have pictures of are only within the Milky Way...if that is indeed the case.

This may also apply to stars themselves. What is the radius in ly of discernibility of (most) stars? Is it beyond the distance to the center of our galaxy (where even in principle individual stars might be discernible but there's too much of a mess to differentiate)?


texts: Objects beyond 13.3 billion light-years away are (apparently) moving away from us at faster-than-light speeds?

texts:
That is, if you use the cosmic distance ladder method, and a value of about 73.5 for the Hubble constant....
But, if you plug in the Planck CMB value of about 67.5, you get a distance of about 14.5 billion light years, beyond which objects seem to be moving away from us at superluminal speeds...
Are those values correct?


texts: A couple of decades ago I read an article (could have been either Astronomy, Space, or Science News) that suggested the contents of the universe may be older than the universe itself.  Of course, that result was so preposterous that it was summarily dismissed.  However, when I read that article, I thought it made sense.  Surely the universe is much more massive than its contents, so as General Relatively predicts, should experience time more slowly.  This conclusion seemed so obvious that a) a bunch of other folks much brighter than I would have thought of it, and b) therefore my conclusion is completely erroneous.  So my question is: why wouldn't the mass of the universe cause it to experience time at a rate slower than its contents as GR predicts?  (There are a couple of questions on SE that are similar, but I do not believe this is a duplicate.)


texts: Can you test solar eclipse glasses with a remote control?

texts:
By putting solar eclipse glasses directly between a remote control IR emitter and device receiver, could some unsafe glasses be detected? My guess is most fake glasses are just blocking visible light at best, while safe ones should block the IR signal as well. It would not guarantee they are 100% safe, but it might be able to detect bad ones.

I know to check for the ISO logo, cert number, and buy from a reputable vendor recommenced by the American Astronomical Society (which I have). It would still be nice to double check pairs of glasses before giving them out to friends and family.


texts: If I have amateur astrophotography image data that might be of scientific value, what should I do?

texts:
I have some image data that may be interesting to someone, somewhere, but I don't know who to contact or notify.

I would imagine it would depend on the type of data, but maybe there is a central place people can contact?


texts: Has NASA confirmed earth now has a second moon orbiting the planet?

texts:
Several sites claim NASA has confirmed a second moon orbiting earth. This is the Asteroid 2016 HO3. One source states The newfound quasi-satellite/moon-2016 HO3, is likely larger than 120 feet (40 m) and smaller than 300 feet (100 m).

http://www.physics-astronomy.com/2016/06/nasa-just-confirmed-that-earth-has-new.html#.V3tdaNIrLIU

Question
Is this claim true circling the internet and can anyone post a link from NASAs offical web site to conclude this is true?


texts: What's the name of outside the cosmic horizon?

texts: The word we use to refer to what's inside the cosmic horizon is the 'universe', so what would you call the empty space outside of the horizon?


texts:
I was taking shots of the 2017 eclipse using my D7200 camera with a 1000 ND filter. As I was going through the pictures after downloading them to a pc, I noticed that there's what appears to be a tiny light source present off to the side of the sun. I have circled it in the picture below. It is also present in the pictures during totality, which I took without the ND filter.

It doesn't seem like a camera or a lens artifact, since it showed up pretty consistently in different shot configurations, but I did not know that a 300 mm zoom with an 1000 ND filter could capture a light source besides the sun and the moon in full day light. If it is not an artifact, what could be the source of this light?


texts: How harmful is Saturn's magnetic field for human life?

texts:
AFAIK, Jupiter has the strongest magnetic field in the solar system, and it's so powerful that I've been told humans only could survive the radiation generated by it in its mooon Callisto, with proper protection.

But how about Saturn? Saturn has the second strongest magnetic field in the solar system. Which moons of Saturn could men survive from the radiation generated by the planet's magnetic field in?


texts: From what distance could the Sun's planetary nebula be seen entirely with free eye?

texts:
When collapsing to a white dwarf, a red giant Sun would emit a planetary nebula. How far from the Sun would that nebula stretch at most?
Also: At about what distance could you see the planetary nebula completely in all its beauty, I mean so that the entire nebula would match into your eye's field of view? The Earth and Mars would be deep within the nebula I guess, as well as the main asteroid belt. If you watched the Sun from Eris' aphelion or Sedna's perihelion, I guess that would be best to observe the entire nebula or ain't I correct? And what would the nebula look like from Proxima Centauri b?
In other words, how close to the Sun's planetary nebula can you be and still see it entirely in its beauty?


texts: Recreating Observer Ecliptic Lat/Lon from JPL Horizons using SPICE toolkit

texts:
I apologise if this is the wrong place to ask. I am trying to recreate part of the calculations by the JPL Horizons system using SPICE. Specifically the Observer Ecliptic Latitude and Longitudes.

I'd here's the inputs.  (1 = RA, 3 = DEC, 31 = Observer Ecliptic Lat/Lon)

Here's the output...

*****************************************************************************************
 Date__(UT)__HR:MN     R.A._(ICRF/J2000.0)_DEC dRA*cosD d(DEC)/dt    ObsEcLon    ObsEcLat
*****************************************************************************************
$$SOE
 2019-Apr-05 00:00     23 18 17.44 -05 37 22.3 94.28997  10.27670 348.4729140  -1.0458502
 2019-Apr-06 00:00     23 20 55.85 -05 32 12.3 102.7090  15.59295 349.1112323  -1.2238769

I've been going through the documentation for SPICE and trying a few bits and pieces appear to be likely functions, such as SPKPOS_C and SPKCPO_C trying to work out how to recreate the above. I'm using de430.bsp to try and get positions to then work out the lat/lon, but I'm getting lost.

I'm wondering if anyone familiar enough with the SPICE toolkit to point me at the functions/methods that will lead me to what I need? Thanks.


texts: If you keep adding electrons or protons to a Neutron star, what would happen? Are there decay processes that convert them to neutrons in the absence of the oppositely charged particle?


texts: Can there be an infinity of humans in the Universe?

texts:
I was reading this question about if the number of stars can or cannot be infinite.
The answer to this question is:

"if the universe is considered infinite, the number of stars can be infinite, otherwise not."

So, I came across a mental problem: If the universe is considered infinite, and therefore we can consider that the number of stars is too, then, we can consider that the number of humans (example) is also infinite, or the number of anything is also infinite?

If the answer is No, please describe why u think this is not a real affirmation.


texts: Can you provide a visual description of the sun rise?

texts: The Sun rises in the east only two times a year. The other days, the Sun rises in the east, but a little bit to the north, or to the south. How can you know the degree of how much north or south it is? Does it vary in different hemispheres?


texts: Approximating density of hydrogen in [observable] universe

texts:
Let universe be completely made from hydrogen. And also we have redshift $z= 6$. with Hubble constant $H_{0} = 2.1941747572815535\times 10^{-18}\:\mathrm{s}^{-1}$. We also know that density of the universe $\rho =\frac{3H^2}{8\pi G}$ ($G= 6.67\times 10^{-11}\:\mathrm{m^3\ kg^{-1}\ s^{-2}}$).

Is the following approach right to find the number density of hydrogen in universe?

First we calculate density $\rho =\frac{3H^2}{8\pi G}$, then we have
$1+z = \sqrt{\frac{1+v/c}{1-v/c}}$. From that we get $v = H\times R$ and from this we calculate $R$, the radius of observable universe. With that, we calculate volume of universe $V = (4\pi/3)R^3$ and we get Mass $M=\rho V$. Then we calculate number of Hydrogen atoms with $N = M/m_{\mathrm{hydrogen}}$. Now we have number density of hydrogen $\rho = N/V$.

Is this solution right? Or we must solve it another way?

The source of question is National Olympiad of Iran which is not available in English so the above is my translation for the question.


texts: The Earth does not have a defined geometric shape. Then how do space scientists who want to put a satellite in orbit model the gravitational field of Earth? Do they assume a single object with minute variations from sphericity, or as a combination of multiple objects of defined shapes? Or, are there different ways of doing it right?


texts: Other than liquid water what factors could make way for life to be on Europa?

texts: I know that Europa has liquid water, but I just don't know if there is anything else on it that could give way for life.


texts: I have a scientific paper on gravity, how do I publish it, and will my research benefit me?

texts: I read a lot about gravity and discovered a hole in it, and I want to publish that hole, but I am not good at mathematics. To show that gap, what should I do? Should I use Newton's equations which are easy in my research, or fight and read about Einstein's equations which are very difficult


texts: Why aren't brown dwarfs the dark matter?

texts: There seems to be not enough matter to account for the fact that the speed of some stars located far from the Galaxy center is almost the same speed as those stars found nearer to the center of the Galaxy, therefore it seems there should be even more matter: so this missing matter is termed dark matter. There is a kind of stellar object that isn't massive enough to produce its own light but is many times more massive than Jupiter, a.k.a brown dwarf. Since it isn't bright enough to be detected by our deep space telescope nor the earth bound counterparts, they should be numerous in galaxies as I believe these objects sits in the middle range between planets and stars. However it seems the scientific community had ruled out this potential dark matter candidate without scanning the unobservable Milky Way galaxy as well as billions other with different light spectrums. Why aren't brown dwarfs considered candidates for the mysterious dark matter?


texts: 70-year star in Talmud confusing sailors?

texts:
I don't know if this question is a good fit for here - it probably wants the non-existent "History of Astronomic Navigation SE", or maybe just "History SE" - but I'll start here:

As described in this article here by Dr Mario Livio:  The Talmud (an 1500-2000 year-old Jewish book of laws and traditions) describes how a Rabbi anticipated hs ship getting lost, because “There is a star that appears every 70 years and induces navigation errors.  I thought it might appear and cause us to go astray.”.

Various theories are put forward to the identity of this star - with the 70-year period suggesting Halley's comet, although another theory posits that it was a variable star (see article for details).

My question is one that the article doesn't seem to address: How could a comet or a variable star confuse sailors? Comets don't even look like stars, so as long as they don't cover up the Pole Star or any other important navigational bodies why should they confuse sailors?


texts: I've hypothesized for many years in my head that dark matter are the gravitational after effects of objects that have exploded. My understanding of physics is that when an object explodes, its center of mass remains in the same location. If that's the case, it seems that the gravity of that center of mass could remain. What observations or theories are there that counteract this hypothesis?


texts: Can any radio telescope be hooked up with the Very Long Baseline Array, or other interferometers?

texts: The Very Long Baseline Array uses both Arecibo and Spectr-R, the world's largest space telescope with 10 m diameter at Lunar distance, as well as many other antennas. Are there any particular requirements on the design for a radio telescope to be used interferometrically? Could for example the now operating Chinese Five Hundred meter Spherical Telescope be used as a part of the VLBA?


texts:
a fresh astronomy enthusiast here.
Recently I've been reading upon how immensely far other stars are from our solar system and how is it possible that we may be seeing stars that are already dead. My question is - if there is a star that is a millions of light years away from us and we are seeing it shine tonight, but in reality that star died long long time ago (perhaps also millions of years ago), is it possible that matter from that star somehow travelled through space to our solar system and at some point in history it collided with the Earth and now we're "made" of this star (made as in that star stuff is present here on Earth)?
To me, I don't see reason why not, because even though matter cannot travel faster than light, my intuition tells me that the answer to my question is yes, it is possible that this could happen, (if only in theory) given enough distance. But maybe I'm not considering some important points which are obvious to more experienced folk.


texts: If all living things like animals require oxygen to live, how can people think that there could be life on other planets such as mars for example ? Living things require oxygen and carbon dioxide, and these planets do not have plants and such to provide this. So why do scientists try to still look for life on these planets ?


texts: Neutrino Modelling in Friedmann Equation

texts:
I'm trying to model neutrinos in the Friedmann Equation. I've covered the case of the Benchmark Model where we have matter, radiation, curvature, and the cosmological constant, Lambda. I know my coding of the Friedmann equation works because I get the correct plots at different parameters, as you'll see attached below.
Including neutrinos, the Friedmann Equation becomes
$$
\begin{eqnarray}
H(z)^2 & = & H_0^2 \Big[ (\Omega_c + \Omega_b) ( 1 + z)^3 + \Omega_\gamma ( 1 + z) ^4 \\
& + & \Omega_{DE} ( 1 + z)^{3(1+w)} + \Omega_k ( 1 + z) ^2
+ \frac{\rho_{\nu, tot}(z)}{\rho_{crit,0}} \Big].
\end{eqnarray}
$$
To solve for the energy density as a function of the scale factor (or redshift), we can solve for the energy density by the following expression for the energy density of a single neutrino species:
$$
\rho_\nu (T_\nu) = \frac{g}{(2\pi)^3} \int
\frac{\sqrt{p^2 + m^2}}{e^{p/T_\nu} + 1} d^3 p.
$$
The energy density critical is $4870$ Mev/m$^3$. Energy density of the single species can be written as function of scale factor by writing the temperature as a function of scale factor. $T$ is simply the expression shown below divided by a:

In equation 17, we can write $d^3p$ as $4\pi p^2 dp$ and $g=2$ for a neutrino species. Another thing to notice is that (17) is written in natural units where $c = h = k = 1$. I've tried to fix the units and no matter what I do, the density parameter of the neutrino species is always very small (order of $10^{-9}$) where it should be between 0.0013 and 0.007 from Ryden, Intro to Cosmology equation (7.54).
I was really hoping someone can help me with the unit conversion from the natural units to the proper units. Everything else I've figured out, I just can't seem to fix the units for equation (17).
Without neutrinos, I get the following plot consisting of various universe models, and they are correct so coding is not the problem. The problem is the unit conversion to proper SI units of (17).

Once I get the neutrinos figured out, I want to see how they affect the universe models. Any help is greatly appreciated!


texts:
When light is emitted by for example a star, that star loses energy - which causes it to reduce its gravity. Then that energy begins a journey for potentially billions of years, until it reaches some other object.

When that light reaches a surface, such as another star or galaxy, it will give that energy to the destination star in the form of heat. This causes the receiver to increase its energy, in turn restoring a sort of balance. It also causes the receiver to emit a minute amount of more light again, almost like a reflection.

It will also excert pressure on the receiving surface once it reaches its destination, be it a star, a rock or anything else.

But while that light is travelling through space, its energy is "unavailable" to the rest of the universe. Naturally I ask the following question:

Will light cause gravity, while it is traveling?

Every single star emits light in every direction, and will eventually reach every other star in the universe. At any single point in the universe, there must be a continous ray of light coming from every single other star in the universe, that has a direct path to that point. Given that all stars on the sky is sending photons that reaches every square centimeter of the earth surface, the amount of pressure should sum up to be quite large.

Is the amount of pressure really neglible, given that every single atom on any surface is receiving light from every single lightsource on the sky?

Based on a calculation found at http://solar-center.stanford.edu/FAQ/Qshrink.html the sun will during its lifetime emit 0.034 % of its total mass as energy. Assuming the sun is average, and that there are about 10^24 stars in the universe, and all of these stars on average are half way through their lifetime, there should be energy amounting to the gravity of about 1.7*10^22 suns distributed throughout the universe.


texts: Which celestial body's length of day is closest to that of Earth?

texts: I know the planets Mars and Eris are among bodies that have the most Earth-like mean solar days (about 24.67 and 25.9 hrs). Among moons there is Mimas of Saturn which has one of the most Earth-like days. Uranus and Neptune also have more Earth-like day lengths than many other known planets or moons. However I don't know of a body in the Earth's planetary system that would have a more Earth-like solar day than Mars (other than the Earth of course). Do you know a planet, moon or asteroid that has a daylength closer to that of Earth than Mars' is?


texts: How does the concept of a universe with no center work?

texts:
I understand basically that the universe is homogenous (looks the same from every point) and I was told in my astro class that it's supposed to have no center, but how that works boggles my mind. I can't get over the feeling that from some point in the universe a civilization could look in a certain direction and see a sign indicative of the direction towards the edge or the center, like younger galaxies or a lack of galaxies moving a certain direction.

EDIT: Thanks to Sir Cumference for highlighting some things that are unclear about my question. Sure, to a certain extent you might be able to think of the universe as being without a center, but if you think of a ball growing larger from nothing, and you pick a random point in the growing sphere, you can always say that the point is further from some points on the sphere than others, or in other words, that there is a direction towards a "center".

Any other details you can give that are related at all would be extremely welcome.


texts: Which moons have cold traps? (i.e. low ecliptic inclination in orbital and rotational axes)

texts:
Both the Moon and Mercury have polar cold traps in polar craters, with signs of volatiles. This is of course not as interesting on a moon in the outer Solar System which consists of volatiles anyway. But suppose that there is some use for very cold real estate shielded from the Sun out there.

Which moons are candidates to have eternally shadowed (or eternally sunlit) regions, and wouldn't they be even colder than those on the Moon? Jupiter and Neptune have less than 2 degrees inclination so their moons maybe are good candidates.


texts:
I'm currently trying to write a C++ library that makes accessing binary data from a JPL ephemeris file easier. So far I sifted through the web, trying to find a good and thorough documentation about what is actually stored in there and how to work with the data.

Does anybody know of a PDF/Book or something similar, that explains anything of that sort?

Thanks in advance!


texts: Taking Hubble's law into account, what is the velocity of the most distant galaxies relative to Earth? Yes, I know that it is actually space that is expanding, (so, it is not their real velocity), but at what speed relative to Earth are they receding from us?


texts: What is the orientation of the M87 black hole image relative to the jet?

texts:
On the image of the M87 black hole released today by the Event Horizon Telescope team, which direction points towards the jet? Is it on the brighter side of the ring?

To resolve ambiguity, I'm talking about the main observed jet, not the fainter counterjet.


texts:
Unlike its sister planet Uranus,Neptune has a substantial internal heat source which gives rise to a dynamic atmosphere & weather.

Wikipedia says:

  As with Uranus, the source of this heating is unknown, but the discrepancy is larger: Uranus only radiates 1.1 times as much energy as it receives from the Sun; whereas Neptune radiates about 2.61 times as much energy as it receives from the Sun. Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Depending on the thermal properties of its interior, the heat left over from Neptune's formation may be sufficient to explain its current heat flow, though it is more difficult to simultaneously explain Uranus's lack of internal heat while preserving the apparent similarity between the two planets.

Have plausible theories been proposed for the source of the heat necessary to keep Neptune's ocean liquid?

Could there be sufficient methane in the atmosphere to produce a significant greenhouse effect?


texts: Which spacecraft would be moving faster?

texts:
If identical spacecraft were orbiting Mars and Earth at identical radii (distances), which spacecraft would be moving faster? Why?

My reasoning is based on equation: a3 = P2 * (M1 + M2). Earth's mass is larger that that of Mars (we can ignore the mass of the spacecraft since it is negligible in comparison that of the planets), and the problem states that a is also the same in both cases. Therefore, for a spacecraft orbiting Earth P (orbital period) should be less than the one orbiting Mars. It implies that it should move faster.

Am I reasoning correctly? Please consider that it is a question from a introductory textbook chapter which is about laws of planetary motions, Newton's law of gravitations, orbits in Solar system etc. So answer should be adequate to this level.


texts: Higgs Bosons in the centre of Black holes or extremely large stars

texts: With the discovery of the Higgs Boson as the mass carrier, what is the possibility at the centre of a black hole being so dense and gravity so infinite that the structure of a proton or neutron are ripped apart into their fundamental elements by these extreme energies that Higgs Bosons are massed together at the centre, while the remaining massless subatomic particles are just outside this layer, similar to an onion  (Similar to a star as more elements fuse together they become more dense, and gravity forces them to the centre again like an onion)